The invention relates to a high pressure mixing head for the production of a mixture of at least two, preferably chemically reactive, plastic components in a mixing chamber which has inlet openings for the individual components and a common outlet opening for the resulting component mixture. An expulsion piston of the same cross section as the mixing chamber, is arranged in the mixing chamber and adapted for reciprocating movement from a retracted position which leaves the inlet openings in the mixing chamber uncovered to an extended position in which the face of the piston is adjacent the common outlet opening, the inlet openings into the mixing chamber being covered by the piston in this later position.
The piston, furthermore, by reason of having the same cross section as the mixing chamber, serves to clear the walls of the mixing chamber of mixture residues as it moves to its extended position.
In a previously proposed mixing head of this type, the inlet openings or ports communicating the mixing chamber with the respective sources of the polymer components open and close alternately and simultaneously without any particular control means having to be provided for the opening and closing, since the inlet openings are respectively opened and closed with reciprocation of the expulsion piston.
The beginning of the injection operation, the so-called shot time, is determined by the beginning of movement of the expulsion piston, something which however in many cases leads to inaccuracy in metering the quantity of the plastics components because at the beginning of its movement the expulsion piston firstly has to be "got moving" out of the one into the other end position. Thus more especially in the case of a substantial elapse of time between two subsequent shots the expulsion piston inclines to "stick" in its particular end position. The consequence of this is that the actual shot time is less than the theoretical or theoretically desirable shot time, so that the metered quantity of the plastics components is too small and this leads to foam products which have to be rejected.